The invention relates to current source inverters (CSI) in general, and more particularly to adjustable speed AC motor drives embodying the same. A current source inverter (CSI), as the name implies, is one where the variable frequency electrical output is a current wave rather than a voltage wave.
A CSI includes a large inductor on the DC-side and a three phase phase-controlled-rectifier (PCR) connected to the AC power line. The rectifier generates variable DC voltage which is converted to a variable current source by a large DC-link inductor. The thyristors of the inverter pass the current from the current source symmetrically to the three phases of the machine while generating a variable-frequency, six-stepped current wave in the CSI. The CSI has many advantages over the voltage-source inverter (VSI): fuseless protection, full four-quadrant regenerative capability with simple structure, rugged and reliable operation, etc. In spite of such advantages, the conventional CSI also has several limitations: lower operating frequency range, bulkiness, cost, high spike voltage at the machine terminals, etc. The shortcomings of the conventional CSI mainly arise from the existence of the leakage inductance of the machine. Energy exchange between the leakage inductance and the commutation circuit is necessary in order to obtain the change from phase to phase of the current of the machine at every instant of commutation. To accomplish this various methods have been used which, however, have the aforementioned limitations typical of the conventional CSI.
For instance, a well-known and widely used CSI is the auto-sequential commutated inverter (ASCI), the schematic of which is shown in FIG. 1. Six thyristors, six diodes and six capacitors are used in the ASCI. The capacitors are used both to commutate thyristors and to take over the energy stored in the leakage inductance of the machine. The diodes play a role in isolating the capacitors from the load and in helping to store energy for commutation.
In the ASCI, the operating range of the frequency and the spike voltages are a function of the load, the capacitance value and the machine leakage inductance. Since the capacitors are discharged and charged by the DC-current flowing from the current source during the time of commutation, the upper limit of the frequency of operation is limited by the light load condition and when the DC current is minimum. On the other hand, the capacitor receives its maximum voltage when at maximum load condition, when the DC current is maximum. In order to increase the operating frequency range, the capacitance value must be decreased and made as small as possible. However, the capacitance value should be increased and made as large as possible in order to decrease the peak capacitor voltage. The compromise resides in a choice of a capacitance value which makes the operating frequency somewhat lower and the voltage ratings of the thyristors and the diodes somewhat higher.
In order to cope with such problems, several modifications have been made in the circuitry. FIG. 3 hereinafter shows such a circuit with a diode, a resistor and an inductor connected in parallel with the main thyristor, whereby the capacitor discharging time is reduced and the operating frequency range is extended by creating with the inductance and the resistor an additional discharging path during the commutation interval. However, the efficiency of the inverter is reduced by the dissipation in the resistor of part of the commutation energy to prevent the capacitor voltage from building up too high as the commutating operation repeats itself.
In order to increase the inverter efficiency as well as to widen the operating range, clamp circuits have been used, as is shown in FIG. 4 hereinafter. In an ASCI with a clamp, the capacitors are used only for the commutation and the peak capacitor voltage is limited by the clamp circuit which absorbs the energy stored in the machine leakage inductance during the commutation interval. This type of circuit operates in a wide range of frequency and within a limited capacitor voltage. However, the clamp circuit tends to be bulky and expensive if a technique is used by which the energy is reconverted to in such a form that it can be passed back into the power line.